[Elecraft] K3: roofing filter configuration settings

[Joe Subich, W4TV]

On 2/10/2016 11:12 AM, john at kk9a.com wrote:
 > This sounds like a great idea. I used Inrad 250Hz filters in my Yaesu
 > FT1000MP Mark-V and they worked great on RTTY but I found the 250Hz
 > setting to be too narrow on my K3S.

The INRAD filters are spec'd for -3dB bandwidth.  This goes way back to
the original "International Radio" as a spin off of the Fox Tango club.
Yaesu (and Kenwood) specified their filters for operational bandwidth -
with two filters (1st and 2nd IF) in cascade.  Thus 250 Hz at - 3dB in
each filter/IF resulted in an overall 250 Hz at -6dB for the two in
cascade.

A *single* 250 Hz filter - typically the 1st IF - was a very good RTTY
filter since it had a 370 - 400 Hz bandwidth and relatively low group
delay (phase distortion) at the "corners".

Unfortunately, that doesn't quite work with the K3 where the DSP is set
to 250 Hz because the DSP has very sharp skirts (high levels of phase
distortion).  However, it will work if the "250 Hz" filter is set to
kick in at 390 Hz in RTTY and the DSP is also set for 390 Hz.  You
achieve a very sharp 390 Hz bandwidth (the DSP "cleans up" the skirts
of the IF filter) with minimum required bandwidth for 45 baud 170 Hz
shift RTTY.

Going back to the original question in this thread ... setting the 400
Hz filter as a 450 Hz and the 250 Hz filter as a 370 Hz filter results
in the sharpest skirts with overall bandwidth (at - 6dB) roughly the
same as the 450/370 settings.  That may not be enough difference to
justify the cost (price and "slot") of both IF filters and is one reason
I prefer the "400 Hz" along with the Elecraft 200 Hz - 5 pole filter
when it is available.  I don't see enough improvement with the "250 Hz"
filter in RTTY to justify both.

73,

   ... Joe, W4TV